This means that a feedoor and a small public box setup = 5 + 1 = 6 plat. Same as the small buybox.

But then 5 + 2 = 7 for a big public box and feedoor setup. Sim has priced the large buybox at 8 plat. This means that it is 1 plat cheaper if you are selling 1000 items or more than 500, to use the feedoor and large public box.